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2 Answers
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apt-get install ntp. If you've never had it installed before, you should be good to go. If you have, and therefore, might have stale config files, you could apt-get remove --purge ntp and apt-get install ntp again to get new config files. Or if you want to keep your config files, I recommend checking that /etc/default/ntp has NTPD_OPTS='-g'. This says to ntpd "try to correct the time, even if it is way off", and make sure at least one good server in /etc/ntp.conf has the iburst option, like:

server 0.debian.pool.ntp.org iburst

This allows a burst of packets to be sent which will get your time synced quicker.

If you ever need to have something wait for time sync before proceeding, run ntp-wait. If, for example, you have a service which shouldn't start until you are sure you have your time corrected. (which you previously might have used ntpdate for)